1. Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating such a device. More particularly, the invention relates to a semiconductor laser device for use in optical data processing and to a method for fabricating that device.
2. Description of the Related Art
A trend in recent years has been for CD-R/RW drives to run at increasingly higher speeds than before. That trend has entailed a growing need for semiconductor lasers producing light in the 780 nm band, which are used by the high-speed drives, to provide greater output. A major constraint on getting the semiconductor laser to be more highly powered is a degradation of its light emitting facet. This type of degradation, called COD (catastrophic optical damage) degradation, stems from defects in the vicinity of the light emitting facet causing optical absorption.
One way to reduce the COD degradation at the light emitting facet is by having a window structure laser with a wide band gap region, i.e., a region where no light absorption takes place, formed on the light emitting facet. One such solution is described illustratively in the Sharp Technical Report (a Japanese publication), No. 50, September 1991, pp. 33–36.
FIG. 14 is a partial perspective view of a semiconductor laser having a conventional window structure. FIGS. 15A and 15B are partial perspective views showing steps of a method for fabricating the semiconductor laser with the conventional window structure.
In FIG. 14, reference numeral 100 represents a semiconductor laser, and 102 denotes an n-type GaAs substrate (in the description that follows, the symbol “n-” stands for the n-conductivity type, “p-” for the p-conductivity type, and “i-” for an intrinsic semiconductor). Reference numeral 104 stands for an n-Al0.5Ga0.5As lower clad layer; 106 for an MQW active layer having an i-Al0.1Ga0.5As well layer; 108 for a p-Al0.5Ga0.5As first upper clad layer; 110 for an n-AlGaAs current blocking layer; 112 for a p-AlGaAs second upper clad layer; 114 for a p-GaAs contact layer; 116 for an i-Al0.5Ga0.5As window layer with a band gap greater than that of the MQW active layer 106; and 118 for an electrode.
The conventional method for fabricating the semiconductor laser sketched above will now be outlined. In FIG. 15A, the lower clad layer 104, MQW active layer 106 and first upper clad layer 108 are epitaxially grown on the n-GaAs substrate 102. With a ridge produced by etching, the current blocking layer 110 is selectively grown. The second upper clad layer 112 and the contact layer 114 are then formed over the ridge and current blocking layer 110. The result of these steps is shown in FIG. 15A.
Thereafter, the n-GaAs substrate 102 is reduced in thickness at the back surface to a thickness of about 100 μm. Laser facets are formed by cleaving and the window layer 116 is formed by crystalline growth. The result of this process is illustrated in FIG. 15B. Forming the electrode 118 on the structure completes the semiconductor laser of FIG. 14.
On the conventional semiconductor laser 100 constituted as outlined above, the window layer 116 is formed by crystalline growth on the cleaved surface following cleavage of the laser facets. This conventional process tends to be complicated because the window layer 116 and electrode 118 need to be formed after the cleaving step.
Japanese Patent Publication No. 2827919, which is equivalent to U.S. Pat. No. 4,809,289, discloses a method for forming a window structure. The method includes forming a first upper clad layer on an MQW active layer, to be topped subsequently with an ion implantation mask pattern, and forming the window structure by disordering the MQW active layer in the vicinity of the laser facet by means of impurity implantation at a low energy level. According to the disclosed method, the degree of disordering must be controlled precisely, otherwise the window effect will not occur, resulting in a semiconductor laser degrading during use.